There is a major need in space and military applications for infrared detectors. A great deal of research has focused on the possibility of using II-VI compounds, such as Hg.sub.x Cd.sub.1-x Te, to fabricate intrinsic semiconductor long wavelength IR detectors. The interest in these materials is motivated by the fact that the alloy changes from semiconducting to semimetallic as a function of composition. Thus, the bandgap, which is determined by the alloy composition, is narrow and can be reduced to zero for appropriate alloy compositions. However, current attempts to achieve IR detection with II-VI compounds have materials problems inherent in their chemistry, as well as immature growth and processing technologies.
The use of III-V compound semiconductors or other material systems would be preferable in view of the more mature growth and processing technologies available, as well as superior materials properties inherent in their chemistry, but IR detection in these materials has only been demonstrated in Al.sub.x Ga.sub.1-x As for wavelengths of 8 to 11 .mu.m. These smaller wavelength detectors were based on IR absorption between the ground state and the first excited state of quantum wells formed by the conduction band discontinuity between the GaAs wells and Al.sub.x Ga.sub.1-x As barriers. The photoexcited carriers tunnel out of the wells and are collected with the aid of an applied electric field.
The wavelength at which the detector operates is determined by the energy difference between the ground and first excited states. The requirement that the excited state be near the top of the well for efficient tunneling places restrictions on the range of alloy compositions and well widths which are suitable. In addition, a design for narrow bandwidth operation requires that the well width be .about.100 .ANG. or greater in order to minimize energy level broadening caused by the inevitable fluctuations in alloy composition, placing a further restriction on growth parameters. Finally, a relatively large well width and/or barrier height is required in order for a well to have an excited state in addition to the ground state.